On Energy Control of Modular Multilevel Converters

Owing to the advancements in the area of power electronics, efficient and flexible ac to dc conversion is made possible, bringing back into focus the idea of the dc power transmission at various voltage levels. Several technical and economical factors advocate for building future distribution grids as dc instead of ac, as well as reusing the existing ac distribution infrastructure and converting it to dc.

Among various ac-dc converter topologies, the modular multilevel converter (MMC) stands out with its high reliability, availability achieved through redundancy, high efficiency due to low switching frequencies, elimination of bulky filters and transformers, and fast transient response. It is modular and scalable, offering the possibility to meet any voltage level using commercially-available semiconductors. For all these advantages the MMC family of converters has already found its application in various dc-ac, ac-dc, ac-ac, and dc-dc conversion tasks.

The MMC typically consists of a high number of submodules (SMs), built of a switching module and a floating capacitor, acting together as a variable voltage source. To ensure a proper operation of the converter, the voltage (energy) within the capacitors should be maintained around predefined values.

This thesis explores different mechanisms of the energy control within the standard dc-ac modular multilevel converter. The energy control mechanisms are identified and different methods for their realization are proposed. With respect to the existing solutions, presented solutions are intuitive, simple to implement, and extendable to different topologies from the MMC family of converters.

Due to its high availability achieved through redundancy, the MMC is nowadays applied in various applications, where a high degree of availability is expected, such as in high voltage dc (HVdc) transmission lines. For these reasons, it is also meant to operate properly under faulty conditions, such as grid unbalances, or a failure of a submodule. The proposed energy control methods are analysed for their application under such conditions, and a control method valid under normal and faulty conditions is proposed.

The modular multilevel matrix converter (M3C) is an ac-ac converter belonging to the family of MMCs. It shares the same need for the energy control as the standard ac-dc MMC. The proposed energy control concepts were analysed for the application in the M3C converter, for its various modes of operations and under faulty conditions. A novel energy control scheme was proposed, ensuring full control over the M3C arm energy content under all conditions.

Apart from the energy control, being the core of the thesis, this thesis also presents the development of an experimental test platform used for testing physical MMC submodules. Prior to being used in the real medium voltage (MV) converter, in-house developed submodules are exposed to the electrical and thermal stresses identical to the ones found in a real